A small gestational sac is not necessarily due to low progesterone. A small gestational sac may be caused by a short period of pregnancy, chromosomal abnormalities in the embryo, exposure to harmful substances, and so on. 1. Low progesterone: the mother’s corpus luteum is not functioning properly, resulting in low progesterone in the body. Without enough support from the corpus luteum, the development of the embryo will be abnormal, resulting in a smaller than normal gestational sac. 2. Short pregnancy time: the time of pregnancy is usually calculated by the last menstrual period, but some women have irregular menstruation, the calculated pregnancy time is longer than the real pregnancy time, due to the shorter pregnancy time, so the gestational sac is small. 3. Chromosomal abnormality of the embryo: if there is a problem with the quality of the embryo, the development will be slower and the gestational sac will be small. 4. Exposure to harmful substances: the mother’s exposure to toxic chemical or physical substances, which has an adverse effect on the development of the embryo, will lead to small gestational sac. There are many reasons for small gestational sacs, and it is necessary to consult a doctor in time for examination to clarify the causes and target treatment.